This information applies to pressure measurement in fluids, such as air, other gasses, and water, as required in a variety of buildings research applications.
Pressure measurements fall into three categories, depending on what the measured pressure is compared to:
- Absolute pressure—the pressure of the fluid compared to a vacuum with no pressure
- Gauge pressure—the pressure of a fluid compared to the air pressure in the immediate surrounding environment
- Differential pressure—the difference between fluid pressures at two locations as determined by particular measurement needs. Differential pressure measurements always take the form of P differential = P (location a) - P (location b). When measuring differential pressures, it is critical to be clear which measurement location is a and which is b. This is especially important when the higher pressure can occur at either a or b (e.g., when measuring pressures across a building enclosure). In such cases, the sign of the result is critical information and cannot be assumed. We recommend using statements like "Pressure a wrt (with respect to) Pressure b" or "Pressure a - Pressure b".
The pressure of a fluid independent of any flow is the "static pressure". A sensor placed so that the moving fluid flow impinges on it will sense an additional pressure ("dynamic pressure") related to the momentum of the flow. See the application notes for guidelines on measuring static and dynamic pressure.
Common units of pressure include Pascals (Pa) for air pressure, inches of water (in H2O) for gas pressure and air pressure, and pounds per square inch (psi) for refrigerants, water, and atmospheric pressures.
|Category||Why measured?||Differential, gauge, or absolute||Typical range||Notes|
|Building internal operating pressure (Poutdoor – Pindoor)||For investigating the pressurization effects of ventilation or HVAC fans, or for characterizing likelihood of depressurization that may cause combustion products spillage. Possibly, for quantifying infiltration driving forces.||Diff||-10 to +10 Pa||Direct exposure to wind may yield much higher values|
|Atmospheric combustion appliance draft pressure (Pvent-Proom)||As an indicator of vent spillage (“backdrafting”) and (though not absolute) of direction of vent flow||Diff||-50 to +20 Pa||Usually measured as vent wrt room|
|HVAC system pressures (Pduct – Proom, or Pduct1-Pduct2)||For estimation of blower flow rates, characterizing system pressure drops, or estimating leakage rates||Diff||-300 to +300 Pa||Note distinction between static and dynamic pressure|
|Refrigerant line pressure||To establish operating conditions of compression cycle refrigeration system||Gauge||0 to 500 psi (or higher)||Requires knowledge of compression refrigeration systems|
|Water line pressure||For estimation of flow rate through a valve or system of known characteristics, or to screen for adequate line pressure in a system||Gauge||15 to 150 psi||Requires “wet” design (internals exposed to water)|
|Gas line pressure||To check proper adjustment and operation, or for estimation of flow rate through a device of known characteristics (e.g., a pilot light orifice)||Gauge||Up to 15 inches of water||Warning: hazard of fire and explosion|
|Barometric pressure||To calculate exact density of air, sometimes needed for precise calculations of fan flow rate, energy transfer, and relative humidity||Absolute||14 to 15 psi|
Note: always select sensors that can withstand the maximum pressure that can be expected in your application.
Sensor and transducer types
Pressure transducers are widely available from many manufacturers, with linear (analog) voltage output or 4 to 20 mA (analog) current output. Some come equipped with visual displays (most often digital LCD displays).
Pressure transducers in general may be sensitive to temperature changes. Consult manufacturer's specifications for model-specific information.